In recent years, the use of die casting and plastic molding has been extended to the manufacture of larger and larger articles and has been extended to the manufacture of articles with closer and closer tolerances.
Such large automotive parts as internal combustion engine blocks and the housings of automatic transmissions are now commonly manufactured with die casting as the first step in formation of the part. Such parts have extensive and complex surfaces with close tolerances; and die casting now permits their formation, eliminating costly machining operations and saving metal. Die casting of such large parts as engine blocks and transmission housings requires massive dies. Extreme pressures are exerted on the liquid metal during the die casting operation, and large amounts of heat are extracted from the molten metals as they change state. The maintenance of dimensional tolerances within limits making such operations economically attractive and the strength to withstand the stresses resulting from high pressures and forces require massive dies. The molds for die casting such large automotive parts as automatic transmission housings, for example, are frequently seven to eight feet (2.1-2.5 meters) tall, seven to eight feet (2.1-2.5 meters) wide, and six to seven feet (1.8-2.1 meters) thick when closed and must be manufactured from high-grade, high-tensile strength steel.
Such molds frequently include one stationary element, one movable element operated by the die-casting machine to close the mold, and several slideable elements, referred to as "slides", that move transversely of the direction of movement of the die-casting machine to provide the mold cavity with intricate and re-entrant surface configurations. The mold slides which slide transversely of the direction of movement of the die-casting machine are generally moved by hydraulic cylinders to their proper position. As the mold is closed by the die-casting machine, surfaces on the slides are engaged by the other mold elements or parts in such a manner that when the mold is closed, interengaging surfaces, such as ramps, on the slides and the other mold parts are clamped and held together by the die-casting machine to prevent the high pressure of the injected, molded metal from moving the sliding mold elements or parts.
A large die-cast part, such as an aluminum housing for an automatic transmission, can be fifteen to twenty inches (0.38-0.5 meters) in diameter and have a length of up to two feet (0.6 meters); and the surface area of the mold cavity exposed to the pressure imposed on the liquid molten metal by the die-casting system can equal several hundred square inches (or over one thousand square centimeters).
In die-casting operations, high pressure is needed to fill quickly the intricate cavities of die-casting molds and avoid solidification of the molten metal as it is cooled by the die. High pressure is also needed to prevent the formation of voids in thick walls formed in the molded part. When the cross section of a mold cavity is designed to provide a relatively thick section of, for example, three-eights of an inch (about one centimeter) or thicker, the molten metal at the surface of the mold cools significantly before the interior and, because of metal shrinkage on cooling, can leave voids within a thick cross section. Thus, in large parts, pressures up to 20,000 psi. (1406 kg/cm.sup.2) are exerted on the molten metal to fill the die quickly and prevent the formation of voids within the thick sections, and the pressure of the molten metal can exert millions of pounds (millions of kilograms) of force on the surfaces of the mold cavity. To prevent the millions of pounds (millions of kilograms) of force exerted by the molten metal against the die surfaces from moving the die surfaces outwardly from their intended position, the die-casting machine is operated to preload the mold prior to the shot of molten metal; that is, by the use of ramp-engaging surfaces on the movable mold parts, the die-casting machine locks the movable parts of the die-casting mold together with the imposition of millions of pounds (millions of kilograms) of force on the ramp-engaging surfaces to hold the movable die-casting elements fixed in their designed, closed position. The force imposed, for example, by a 350-ton (317,000 kilogram), die-casting machine, which is not among the largest of such machines, can reach seven million pounds (3,170,000 kilograms) and this force in larger machines, such as 3,500-ton machines can reach 70,000,000 pounds (31,700,000 kilograms). Such high forces, if imposed on misaligned or jammed partially closed dies, can deform and break mold surfaces and the mold-operating surfaces of the die-casting machine.
Die-casting molds for such large parts must, therefore, be designed to withstand such extreme forces and to use the extreme forces imposed by the die-casting machine to prevent movement of the mold slides outwardly as the molten, die-casting metal is injected into the mold under pressures of several tons per square inch (several hundred kilograms per square centimeter).
Die-casting has become desirable as a manufacturing method for parts such as automobile engine blocks and transmission housings because it can produce intricately shaped parts to close tolerances. Thus, die casting can provide such parts with operating strength and intricately shaped surfaces without extensive and expensive machining operations. Such parts are, however, long and have wall thicknesses designed to take advantage of the economy of die-casting operations. Misalignment of the mold parts, due, for example, to warping of the mold, misalignment of the mold on the molding machine, or non-parallelism in the molding machine platen surfaces or their direction of the movement, can reduce wall thicknesses and distort part surface dimensions to unacceptable limits and result in a substantial waste of die-cast parts. Extensive time is required to inspect such complexly shaped parts completely, and failure to detect poorly cast parts before machining can result in further waste.
Furthermore, as indicated above, if the movable mold parts, including the transversely moving mold slides, are not properly aligned as the mold elements are closed to form the cavity, and if the die-casting machine imposes its millions of pounds (millions of kilograms) of force on misaligned mold parts, the force imposed by the die-casting machine may break the mold or parts of the molding machine which operate the mold. Such misalignments can result from undetected trapped flash or dies that become worn through use or operation under high temperatures and stresses. Aluminum has a high latent heat of fusion, reported to be 76.8 gram calories per gram; and a fifty-six pound (over 25-kilogram) aluminum casting, which is typical of aluminum engine blocks and automatic transmission housings, requires the dissipation in the mold of over 7,700 btus (about 2000 kilogram-calories) every minute or two. Although such large die-casting molds are generally cooled by water forced through cooling passages drilled within the mold elements, the large die-casting molds frequently rise in temperature to 300.degree. to 600.degree. F. (150.degree. to 315.degree. C.) and operate at such elevated temperatures. Such high temperatures, of course, prevent human operators from working too close to the mold surfaces because of the danger of burns and other injuries.
Misalignment of die-casting mold elements, failure of the die-casting machine to provide parallel platen surfaces and movement perpendicular to the platen surfaces, and wear of the die elements all cause similar problems in conducting plastic molding and die-casting operations to manufacture smaller, high-precision parts, notwithstanding the substantially reduced forces presented by such operations. Where plastic molding and die-casting operations are used to manufacture small precision parts having intricate surfaces with close tolerances, misalignment of the mold surfaces and failure of the mold surfaces to close correctly will result in parts that fall outside the close dimensional design tolerances and must be rejected.